The existence of particles in a gas sample is an ongoing concern for numerous applications. Particles in the atmosphere of a workplace can be a hazard to the employees. Particles in an electronics manufacturing facility can damage or destroy the components being manufactured. Nuclear, biological, or chemical (NBC) warfare agent particles are harmful to anyone who comes in contact with them. Methods and apparatus for detecting and identifying these particles have thus been developed to address these problems.
One method of detecting particles in a gas sample takes advantage of the well-known fact that particles in the atmosphere scatter optical radiation. In addition, some biological particles emit fluorescent radiation when they are excited by radiation at particular wavelengths. This has lead to the development of systems to detect and identify particles in a gas sample by measuring the radiation that the particles scatter and radiate when exposed to light. However, these systems have generally used long path optical beam systems to detect the particles. The particles scatter the incident radiation, and in some cases emit fluorescent radiation, in all directions. Typically, the light scattered and emitted substantially perpendicular to the source light path is measured. Existing detection systems place a single or multiple detectors around the sample area, only detecting the radiation that is scattered or emitted into a relatively small section of the total sphere. This forces the system to input a proportionally greater amount of radiation to ensure that a measurable level of scattered and emitted radiation reaches the detection electronics. This inefficiency has resulted in expensive detection systems that require a great deal of power to operate.
An example of such a system is disclosed in U.S. Pat. No. 5,920,388 (Sandberg et al.). This patent discloses an apparatus for detecting and identifying particles in a gas sample. The device disclosed uses a laser to irradiate a sensing region. Scattered and emitted light is received by detectors subtending a relatively small part of the unit sphere. Thus, the laser must be operated at a high power level to detect small concentrations of a substance. Further, the laser source can only irradiate one sample section, requiring a separate laser source for every sample area to be tested. This increases the size and cost of the detection system.
A device for counting particles is disclosed in U.S. Pat. No. 5,642,193 (Girvin et al.). The device is typically used to determine the air quality for industrial sites, such as clean rooms. The device comprises a laser with a sample volume in the path of the radiation. A single detector measures the intensity of the light scattered towards it. This patented device requires a high power level to ensure that enough light is scattered towards the detector, such that the detector can discriminate the scattered light over the noise level. Further, only one sample space can be irradiated, requiring a separate laser for each sample space to be tested.
U.S. Pat. No. 4,798,465 (Knollenberg) discloses a device for detecting the size of particles in a gas sample. The device disclosed is used to determine the contamination level of clean rooms. The structure of the device disclosed is similar to the structure of the device disclosed by Girvin et al. It has the same drawbacks: high cost and size, and the requirement of a separate laser source for each sample space to be measured.
Another example of a long path system is disclosed in U.S. Pat. No. 5,527,085 (Ulich et al.). This patent discloses a lidar system comprising a laser that irradiates a target and a telescope that receives backscattered and emitted radiation. The device then identifies the composition of the target based on the radiation received. This device can be mounted in an airplane and flown over a target. The distance to the target dramatically reduces the power received by the telescope, requiring a great deal of radiation to be emitted by the laser to achieve a measurable return at the telescope. As with the other apparatuses, each detection system requires a separate radiation source.
Clearly, then, there is a longfelt need for an apparatus that can detect particles in a gas sample that is lower in cost and size than long path optical detection systems.